1. Field of the Invention
The present invention relates to the field of soundproofing of an aircraft turbomachine and, more particularly, soundproofing upstream to an air blower for a turbomachine.
2. Description of the Related Art
A turbomachine such as an engine turbojet or an aircraft turboprop typically comprises an annular casing for the air blower, axially extending along the turbomachine axis, in which an air blower is mounted, i.e., a rotor disc comprising large size blades. In operation, an airflow circulates from upstream to downstream within the turbomachine. For an aircraft propulsion, the turbomachine is mounted inside an engine nacelle integral with an aircraft wing. Typically, the engine nacelle comprises an annular inlet duct which is mounted upstream the air blowing casing. Referring to FIG. 1 representing a cross sectional half-view of a turbomachine 1 having axis A, soundproofing panels 21, 61 are traditionally mounted inwardly to the inlet duct 2 and to the air blower casing 3 to limit noise annoyance upstream to the turbomachine 1. In the simplest case, the soundproofing panels 21, 61 consist of a honeycomb construction known from person skill in the art. To extend the sound attenuation to a large frequency range, an acoustic panel can comprise several porous layers inserted between honeycomb layers. These acoustic panels are suited to weaken soundwaves coming from the turbomachine 1 and, in particular, soundwaves coming from the air blower 8 of the turbomachine 1. The soundproofing panels 21, 61 are located upstream to an abradable layer 31 of the air blower casing 8, located just in front of the blades of the air blower 8.
As shown in FIG. 1, the inlet duct 2 and the air blower casing 3 comprise each their own soundproofing panels 21, 61 so as to form an internal continuous soundproofing skin. In particular, the air blower casing 3 traditionally comprises, at its circumference, several soundproofing panels 61 end-to-end mounted and integral with the air blower casing 3 by means of fastening screws 6 radially extending with respect to the axis A of the turbomachine as shown in FIG. 1. Such a fastening mode presents drawbacks because it requires to use many fastening screws 6 which, on one hand, compromise the mass of the turbomachine 1 and may damage the blades of the air blower 8 in case of loss or break. On the other hand, the more or less wide interface regions between the panels infer an alternation of smooth and treated regions near the air blower 8 (forming discontinuities of acoustic impedance), which give rise to increases of the noise levels of the turbojet engine for some engine speeds. Additionally, the implementation of the numerous fastening screws on the panels requires to provide for densified regions inside them which impair the acoustic efficiency.
To limit the number of fastening screws 6, the patent application FR 2 935 017 to SNECMA Corporation suggested to lengthen the air blower casing 3 and to attach a single soundproofing panel to the air blower casing 3 and to the inlet duct 2. In practice, the maintenance of such an acoustic panel is difficult to operate since it is necessary to immobilize the turbomachine to proceed to the withdrawal of the damaged panel by a new panel. Besides, if only the downstream part of the panel is damaged, the whole panel must be replaced, increasing the maintenance costs.